1. Field of the Invention
This invention relates to an improved electrical insulating oil which contains a monoolefin and/or diolefin having three aromatic rings, and electrical appliances impregnated with the same oil. More particularly, the electrical insulating oil of the invention is suitable for use in oil-filled electrical appliances such as oil-filled capacitors, especially those in which at least a part of dielectric or insulating material is made of plastics.
2. Description of the Prior Art
In recent years, the requirements for the performances of oil-filled electrical appliances such as capacitors are quite severe. They are required to be small in size, resistant to high voltages and long in life.
In order to meet these requirements, plastics having excellent dielectric strength are used for a part of or for the whole of dielectric materials or electrical insulating materials of oil-filled electrical appliances. That is, in oil-filled capacitors, plastic films made of polyolefins such as polypropylene and cross-linked polyethylene; polyester, polyvinylidene fluoride and polycarbonate are employed together with or in place of the conventional electrical insulating paper.
It is desired, accordingly, that some measures are taken also with respect to the electrical insulating oils that are used for impregnating these electrical appliances.
However, the conventionally used electrical insulating oils such as polybutenes and alkylbenzenes are not good in impregnating property to plastics. Furthermore, recently proposed diarylalkanes, alkylbiphenyls and alkylnaphthalenes are not always satisfactory because they cause the plastics to swell to some extent. In addition, further improvement in their impregnating property is demanded. In the case that an electrical insulating oil is not good in impregnating property to plastics or it causes the plastics to swell, voids (unimpregnated portions) are formed among film materials and electric fields converge on the voids. Accordingly, the corona discharge (partial discharge) characteristics such as corona starting voltage (CSV) and corona ending voltage (CEV) are lowered.
Disclosed in U.S. Pat. No. 4,347,169 are electrical insulating oils comprising diarylalkanes and linear unsaturated dimers of styrenes, and oil-filled electrical appliances impregnated with the same. The olefins disclosed here are only the compounds having two aromatic rings. These compounds are quite different from those of the present invention. In addition, when the compounds in the above reference are used for impregnating capacitors in which metallized films are wound, the performances of the impregnated capacitors are not always satisfactory.